The biggest reason why Einstein failed to unite his theories of relativity and the theory of electromagnetism is his lack of a complete understanding of the principle of relativistic invariance.

Most investigators can recall that the physical concept of mass is not a variable that holds relativistic invariance. This unwanted discovery led to two distinct definitions of mass: (1) rest mass, and (2) relativistic mass. Although Lorentz did discuss electromagnetic mass in his book “The theory of Electron” he did not give its connection with the truly relativistic invariance of electric charge. That is the key point. Why does an electron at rest or an electron in motion approaching the speed of light has the same value of electric charge? Any particle with mass produces a gravitational force. Likewise any particle with electric charge produces a Coulomb force of electrostatic electricity. In quantum electrodynamics, the interactions of moving charges with their antiparticles produce pure energies as photons of light. Conversely, the interactions of photons produce equal numbers of electrons and positrons. Still as observed in repeated experiments electrons or positrons of greater values of electric charge never materialize. Clearly, their electric charge stays the same before and after each interaction. This relativistic invariance of electric charge holds in weak nuclear interactions between leptons. It also holds between quarks and leptons in strong nuclear interactions, which suggested the relativistic invariance of color charges as well. It can be asserted that relativistic invariance plays a much important role for a quantum theory of the space-time continuum. A complete understanding of relativistic invariance will make it easier to find a theory of everything.